One of the major disadvantages of the current state-of-the-art electro-mechanical energy scavengers for some civil, mechanical and biomechanical applications is their narrow-band frequency response which is concentrated around high frequencies. For instance a vibration based scavenger with an overall volume limited to less than 5 cm3 will exhibit a resonant frequency in the range of 50-300 Hz. In literature several broad-band energy scavenging techniques have been proposed that can extend the operating frequency range. However, many real-world processes span frequencies and acceleration levels where its intrinsic energy cannot be scavenged using these technologies and hence have been largely unexplored to date. Some examples of these processes include fundamental vibration modes in large civil structures that span frequencies less than 5 Hz. Likewise changes in physical processes like temperature and pressure variations between day and night induce a stress/strain response in structures that occur at frequencies lower than 1 mHz. In biomedical engineering, changes in in-vivo strain levels during bone-healing and spinal fusion processes span from a few days to up to a few months. In all these processes monitoring the evolution of quasi-static strain is important and could provide significant benefits. For instance the history of mechanical loading inside different structures could be used to predict the life expectancy of the structure. A more challenging prospect and the main focus of this disclosure is to design battery-less sensors that can self-power for harvesting energy directly from these quasi-static processes.
The technical challenge is illustrated for a piezoelectricity driven self-powered sensor whose generic architecture is shown in FIG. 1A. The sensor comprises of a piezoelectric transducer that powers a minimal set of electronic modules by harvesting energy from ambient strain variations. Typically the sensor electronics comprises of: (a) a rectification module to extract the energy from the transducer; (b) a triggering module that detects events of interest; and (c) a data-logging module that records events either on a non-volatile memory or using remote data transmission. When the piezoelectric transducer is excited quasi-statically (as shown in FIG. 1B by the triangular wave) the load voltage generated by the transducer is approximately constant as shown in FIG. 1B. Assuming a nominal sized piezoelectric transducer, the power levels that can be scavenged from mHz strain-signal would be in the order of pico-watts. Unfortunately, the majority of the charge generated by the transducer is lost as leakage through the electronics (for example through diode leakage) and the residual energy is insufficient to drive the rest of the sensor modules (triggering and data-logging modules).
This section provides background information related to the present disclosure which is not necessarily prior art.